Understand where you're coming from, but looks to me as if Radials in 130/80 will fit, a 140/80 should fit, and a 150/70 could fit, depending on wheel offset, and that's without modding the swinging arm, which as it is so long is desperately calling to be braced
A, Sorry, but disagree, in my experience Radials can transform the handling of a number of 80's bikes. OK with those long leading axle forks its never going to be a VFR, but that standard 16" comstar back wheel and x/ply tyre setup is lethal, especially in the wet. That is, unless crossply's have come on a heap in the last 7 years
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Anyway, no worries, I'll take my trusty engineers rule to a couple of meets and see what I can size up
MC